Length Of Separation Zone Research Articles

Simulations of Compression Ramp Shock Wave/Turbulent Boundary Layer Interaction Controlled via Steady Jets at High Reynolds Number

Shock wave/turbulent boundary layer interaction (SBLI) is one of the most common physical phenomena in transonic wing and supersonic aircraft. In this study, the compression ramp SBLI (CR-SBLI) was simulated at a 24° corner at Mach 2.84 using the open-source OpenFOAM improved delayed detached eddy simulation (IDDES) turbulence model and the “Rescaling and Recycling” method at high Reynolds number 1.57×106. The results of the control effect of the jet vortex generator on CR-SBLI showed that the jet array can effectively reduce the length of the separation zone. The simulation results of different jet parameters are obtained. With the increasing jet angle, the reduction in the length of the separation zone first increased and then decreased. In this work, when the jet angle was 60°, the location of the separation point was x/δ=−1.48, which was smaller than other jet angles. The different distances of the jet array also had a great influence. When the distance between the jet and the corner djet=70 mm, the location of the separation point x/δ=−1.48 was smaller than that when djet=65/60 mm. A closer distance between the jet hole and the corner caused the vortex structures to squeeze each other, preventing the formation of a complete vortex structure. On the other hand, when the jet was farther away, the vortex structures could separate effectively before reaching the shock wave, resulting in a better inhibition of SBLI. The simulation primarily focused on exploring the effects of the jet angle and distance, and we obtained the jet parameters that provided the best control effect, effectively reducing the length of the CR-SBLI separation zone.

Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences

This study investigates the distributions of velocity and Reynolds stress in an artificial pool-riffle sequence for two entrance slopes of 5° and 20° in a laboratory and compares the results with a similar pool-riffle sequence in a gravel-bed river in central Iran. Quadrant analysis is applied to find out the contributions of different events on turbulent flow structures. At an entrance slope of 5°, the velocity distribution follows an exponential trend without flow separation, but for the entrance slope of 20°, the velocity distribution indicates a S-shaped pattern with flow separation. Variation of entrance slope does not influence the convex form of Reynolds stress (RS) distribution, but it influences the location of the maximum value of RS and the flow separation zone length. The results reveal that outward and ejection are the dominant events for both field and laboratory settings. The sweep displays a decreasing trend from near the bed toward the water surface, however, the outward contribution indicates an increasing trend from the bed toward the water surface. The agreement between laboratory and field results in velocity and Reynolds stress will help the river engineers to better manage complex fluvial processes. Bursting process events depend on the aspect ratio for the same entrance pool slope. The agreement in the results of velocity and Reynolds stress distributions and the dominant events of bursting process in the laboratory and field are encouraging for better restoration of rivers and decreasing the cost of projects.

Effect of cavity pressure on shock train behavior and panel aeroelasticity in an isolator

The flow characteristics of shock train in the isolator play an important role in the overall performance of the scramjet. Although several studies have concentrated on understanding this phenomenon in rigid isolators, few works have focused on methods to control it. The current study proposes a new concept shock train control strategy based on the aeroelastic effect of the flexible panel. An in-house developed code was used to solve the compressible Navier–Stokes equations and the geometric nonlinear equations of the panel, where the conventional serial staggered algorithm was adopted for the two-way fluid–structure interaction. Then, we numerically investigated the effect of cavity pressure on the dynamic behavior of the panel, location, and structure of the shock train, separation zone, and performance of the isolator. The results show that the dynamic response of the panel subjected to the different cavity pressure can be characterized into three states: static stability state, high-frequency second-order limit cycle flutter state, and multi-frequency periodic flutter state. The panel flutter mainly presents an approximately second-mode pattern for the limit cycle flutter state and a first-order vibration mode for the periodic flutter state. With increasing cavity pressure, the average value of shock-train head location moves downstream significantly, while the general trend of separation zone length on top and bottom walls becomes smaller. The flexible panel with the high-frequency second-order limit cycle flutter state can increase the total pressure recovery coefficient with the smaller side load and outlet flow distortion, reduce the averaged separation length, and make the shock-train head move downstream. This is due to the isentropic compression and expansion waves induced by the vibration and deformation of the flexible panel.

Numerical investigation of the multiphase flow patterns and removal effect in a large dissolved air flotation

Abstract The standard k–ε model coupled with the mixture model was used to study two-phase flow in a large dissolved air flotation (DAF) unit. The numerical results can simulate fairly well the velocity vectors and air volume fraction distribution data of a DAF unit from the literature. The typical DAF structure parameters were analyzed in detail to investigate their predicted influences on the internal flow structure and removal effect. The simulations indicated that the short length of the separation zone was not conducive to the formation of a stratified flow pattern, and the turbulent kinetic energy at the bottom of the separation zone increased as the length decreased. With the increase in the height of the DAF tank, the horizontal flow structure in the separation zone would be disrupted, and the distribution range and the intensity of the turbulence kinetic energy increased. Further analysis showed that the formation of horizontal stratified flow facilitated the removal of bubbles, and the formation of stratified flow is related to the size of the DAF unit. Detailed analyses showed that the reduction of DAF height and the increase of separation zone length were beneficial to improve the bubble removal efficiency. Finally, a theoretical analysis was carried out to study the relationship between DAF parameters and the removal effect. The results revealed that when the horizontal flow structure was not destroyed and stratified flow occurred, the bubble removal efficiency was positively linearly related to the length of the separation zone. The removal efficiency increases as DAF height decreases.

ОЦІНЮВАННЯ ГЕОМЕТРИЧНИХ ПАРАМЕТРІВ ПОЛЮСНОЇ СИСТЕМИ МАТРИЦІ ЕЛЕКТРОМАГНІТНОГО СЕПАРАТОРА

Purpose. The choice of the geometric dimensions ratios of system of matrix poles of electromagnetic polygradient separator to increase productivity with maintaining the reliability of extracting of ferromagnetic impurities from bulk material.Methodology. To solve the dynamic problem of motion of a ferromagnetic body in the working gap of pole system of matrix of polygradient separator under the influence of an external magnetic field the known methods of solving linear inhomogeneous differential equations are used. To confirm the reliability of obtained results the method of experimental research is used.Findings. The formulation of dynamic problem of movement of ferromagnetic body in the working gap of plate pole system of matrix of polygradient separator is carried out. Parametric equation for the trajectory of ferromagnetic body removal and a calculated relation connecting the main geometric dimensions of the system of matrix poles are obtained. The calculation results are confirmed experimentally and by operating practice of known magnetic separating devices.Originality. The mathematical description of working process of a polygradient electromagnetic separator with a plate matrix was further developed, which made it possible to obtain an analytical expression that takes into account the main geometric dimensions of the working space of matrix of separator.Practical value. Accounting of obtained analytical dependences between the length of separation zone and air gap, which characterizes the thickness of the separated material layer through which the ferromagnetic body must pass during the separation process, will ensure the necessary purity and productivity of separation.

Arterial pharmacokinetics in a patient-specific atherosclerotic artery-a simulation study

Of concern in the paper is a numerical study of endovascular drug delivery in a patient-specific atherosclerotic artery through a mathematical model in which the luminal flow is governed by an incompressible vis- cous Newtonian fluid, and the transport of luminal as well as tissue concentration is modeled as an unsteady convection-diffusion process. An image processing technique has been successfully adopted to detect the edges of the computational domain extracted from an asymmetric (about the centerline of the artery) patient-specific atherosclerotic artery. Considering each pixel as a control volume, the Marker and Cell (MAC) method has been leveraged to get a quantitative insight of the model considered by exploiting physiologically realistic initial, boundary as well as interface conditions. Simulated results reveal that the number as well as the length of separation zone does increase with increasing Re, and the near-wall velocity contour might be important for estimating the near-wall residence time for the pool of drug molecules available for tissue uptake. Results also show that the more the tissue porosity and interface roughness do not necessarily imply the more the effective- ness of delivery, even though they enhance the averaged concentration in the tissue domains, and also the area under concentration diminishes with increasing Peclet number. Thus, the tissue porosity, the Peclet number and various geometrical shapes (interface roughness) play a pivotal role in the dispersion and the effectiveness of drug delivery. GANITJ. Bangladesh Math. Soc.41.1 (2021) 62-77

Study on axial-flow hydrocyclone for in-situ sand removal of natural gas hydrate in the subsea

Natural gas hydrate (NGH) has become the most potential emerging green energy known in the 21st century due to its characteristics of wide distribution, abundant reserves and clean combustion. This study designs an axial annulus in situ hydrocyclone desander (AAIHD) based on drilling instruments in order to resolve the serious problem of sand production during solid fluidization of NGH. The effect of the inlet flow rate and separation zone length on the sand removal efficiency of the AAIHD is tested through experimental research. The results indicate that AAIHD has a higher separation performance when the separation zone length is L/D=12.4 and the inlet flow rate is in the range of 10 m3/h to 25 m3/h, and the maximum separation efficiency reaches 77.4%. The purpose of this study is to achieve in-situ sand removal and the backfilling of sand slurry in addition to facilitate the advancement of solid fluidized exploration technologies.

Numerical Simulation of the Interaction between a Shock and the Boundary Layer on a Flat Plate in Motion

The results of numerical simulation of the interaction between a shock and the laminar boundary layer on a flat plate in motion in supersonic perfect-gas flow at Mach number М∞ = 3 are considered. The shock is preassigned using the Rankine—Hugoniot boundary conditions, which corresponds to a shock wave produced by a wedge with a given semi-vertex angle in an inviscid gas flow. The simulation is based on the numerical solution of the time-dependent, two-dimensional Navier—Stokes equations by time marching to steady state. The numerical results are verified by means of comparing the results for separation flow past a flat plate at rest with experimental data. The numerical data are used to investigate the effect of the plate velocity of the separation flow structure and the basic laws governing the problem. It is shown that the motion of the plate downstream diminishes the separation zone length, whereas the opposite motion leads to its increase.

Study on Flow Characteristics of Three-Dimensional Backward-Facing Step at High Reynolds

Fan, J.; Yan, C.; Lei, P.; Wang, C., and Liu, G., 2020. Study on flow characteristics of three-dimensional backward-facing step at high Reynolds. In: Gong, D.; Zhang, M., and Liu, R. (eds.), Advances in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 106, pp. 522–525. Coconut Creek (Florida), ISSN 0749-0208.Based on FLUENT, the three-dimensional Realizable k-e turbulence model is used to simulate the flow over a backward-facing step at 40 Reynolds numbers between 500 and 80,000, and the results are combined with the results of particle image velocimetry experiments to verify the reliability of the model. The three-dimensional flow characteristics of the flow over a backward-facing step at Re = 25,822 are analyzed. The results show that in the two-dimensional aspect, the numerical simulation results are in good agreement with the experimental values, the separation and reattachment zone length change curves are consistent, the flow field structure is similar, and the model reliability is high. In three dimensions, slight differences occur in the flow distribution of the sections perpendicular to Z axis, and the closer the section is to the wall, the greater the change of flow is. Along the Z axis, the velocity and turbulent energy of water flow are unevenly distributed in the spreading direction, showing strong three-dimensional flow characteristics.

Effect of the wall properties on the shock wave / laminar boundary layer interaction

The CFD calculations have been performed based on RANS approach with the Menter transition model [1]. The boundary layer developing on a flat plate was considered. To generate the incident shock wave, a wedge was installed over the plate. The wall was represented by one cm thick solid flat plate made from aluminum and polyacetal. Flow parameters were M∞=1.41, P0=0.7 bar, T0=293K. As a result of solving the conjugate heat transfer problem, the SWBLI flow was studied. The temperature and pressure distributions on the wall for different surface materials were obtained. It was found that the streamwise heat fluxes inside the wall can lead to a change of the separation zone length.

Hydrodynamics of river-channel confluence: toward modeling separation zone using GEP, MARS, M5 Tree and DENFIS techniques

Hydrodynamics of confluence as a natural component in river-channel networks is complicated. Having sufficient knowledge about erosion and sedimentation in the river-channel confluences requires determination of the separation zone dimensions. In this paper, we estimated dimension of separation zone (i.e. length and width) by utilizing data driven techniques: gene expression programming (GEP), M5 Tree model, multivariate adaptive regression spline (MARS), and dynamic evolving neural-fuzzy inference system (DENFIS) trained with hydraulics input variables including discharge ratio (Qr), downstream Froude number (Fr3), and side slope angle of main channel ( $$\sin \,\theta$$ ). Different splitting scenarios, 50–50%, 60–40% and 75–25%, for train-test parts after randomizing, were considered achieving more robust evaluation of the models. The estimated dimensionless length and width (i.e. $$\frac{L}{{B_{3} }}$$ and $$\frac{H}{{B_{3} }}$$ ) were compared with experimental results. In order to measure the accuracy of various models, different statistical criteria including root mean square error (RMSE), Nash–Sutcliffe model efficiency coefficient (NS), correlation coefficient (R), mean absolute error, Legates and McCabe index and Willmott Index of Agreement have been used. For 50–50% train-test scenario, there is a slight difference among the DENFIS, MARS and M5 Tree models while the GEP has the worst accuracy in estimating length of separation zone. The results indicated that by increasing the percentage of training data (i.e. from 50 to 75%), the accuracy of models in term of RMSE was improved as 22, 12, 14, and 15% for the DENFIS, GEP, M5 Tree and MARS models, respectively. Regarding estimation of separation zone width, adding $$\sin \,\theta$$ variable as input considerably increases the models’ performances. The results revealed that M5 Tree is more sensitive to data number in training phase.

Chiral separation of intact amino acids by capillary electrophoresis-mass spectrometry employing a partial filling technique with a crown ether carboxylic acid

An enantiomeric separation method for underivatized free amino acids (AAs) using a partial filling technique with CE-MS was developed for the determination of D-AAs in vinegars. A typical chiral separation method was performed with different concentrations of (18-crown-6)-2,3,11,12-tetracarboxylic acid (18C6H4) dissolved in water or formic acid as the background electrolyte. Seventeen AAs, excluding proline and asparagine, were separated, showing chiral resolution values (Rs) ranging from 0.5 to 21.0. These results included baseline separations of 11 AAs, the peaks of which were observed as the ions [AA+18C6H4+H]+. The migration order of the chiral AAs was also evaluated, and the L-AAs migrated faster than the counterpart D-AAs except for serine, threonine and methionine when using (+)-18C6H4. To reduce contamination of the ESI source by the nonvolatile chiral selector and improve the ionization efficiency in partial filling technique, the separation zone length was adjusted to 70% of the capillary, which was filled with 30 mM 18C6H4 in water. This method showed a similar separation efficiency as the typical method, and the separated AA peaks were observed as free AA ions, [AA+H]+. The optimized method provided limits of detection (LODs) ranging from 0.07 to 1.03 μg/mL and good linearity (R2 > 0.99) up to 50 μg/mL for DL-AAs. The developed method was utilized to determine DL-AAs in vinegars with a simple pretreatment process. It may be extended to sensitive AA analysis in the determination of minor enantiomeric impurities in the major component.

Flow Pattern Around Attractive, Vertical, and Repelling T-Shaped Spur Dikes in a Mild Bend Using CFD Modeling

This piece of research presents flow pattern around a T-shaped spur dike in repelling, attractive, and vertical positions in a 90° mild. To this aim, SSIIM CFD software, one of the most powerful software applications in hydraulic engineering field, was employed. In this research, Navier–Stokes equations and k–e turbulent model were used for solving the flow field. The numerical results were verified using the experimental data. The comparison between the results of numerical simulation and experimental data collected by ADV velocimeter indicated a desirable agreement. The numerical results showed that the dimensions of the vortex at the downstream side of the spur dike reduced by increasing the angle of the spur dike. Moreover, the secondary flow strength value was the maximum for attractive and then repelling modes. Variation of the secondary flow strength for repelling, attractive, and vertical modes ranged from zero to 45, 35, and 25% respectively. Although the range of the secondary flow values in vertical mode for both numerical and experimental models was identical and about 0–25%, less than a 5% error between these two models demonstrated that the amount had been reasonable. By determination of flow separation zones, it was observed that, for all the three modes, the separation zone length increased by increasing the elevation. In addition, the lengths of the separation and reattachment zones were equal to 1.2–2.7 and 1.57–4.18 times the spur dike wing length.

Variation of hydraulic parameters with different wing of a T-shape spur dike in bend channels

In the present research, by using a numerical model, some analyses were performed on flows around a T-shape spur dike and a support structure located at its upstream under different wing to length ratios of T-shape spur dike in the order of 0.25, 0.50, 0.75 and 1.00. In order to verify numerical model, physical model data were used in presence of a single T shape spur dike. Results from numerical model are desirably in agreement with those of physical one because the regression between both data is 0.86 up to 0.92. In this research, all hydraulic parameters of flows, streamlines and dimensions of flow separation zones were studied in order to select the most practical model. Increased W/L results in 7%–12% increase in the length of flow separation zone and in 2% increase in the width of this zone compared to W/L=0.25.

Separation and energy efficiency of packed apparatuses for purifying gases from aerosols

A mathematical model of the turbulent transport of liquid aerosol particles has been considered using an approach in which the transport of a finely dispersed phase is conditionally described as a diffusion process with mass-transfer theory equations. The main parameter in this formulation is the coefficient of the velocity of particle transport (turbulent migration), which is an analogue of the mass transfer coefficient. The use of flow pattern models with a bulk source of the mass of sedimenting particles in a random packed bed and wire-mesh demisters is demonstrated. The equations for calculating the separation zone length and the process efficiency have been derived with allowance for the inlet section and the back mixing of the gas flow. Some calculation results are given for the aerosol separation efficiency and the energy coefficient compared with experimental data.

Flow and scour pattern around submerged and non-submerged T-shaped spur dikes in a 90° bend using the SSIIM model

ABSTRACTThis article studied the effect of submergence percentage of the T-shaped spur dike on scour and flow pattern in a 90° bend. Three submergence percentages of 0, 15 and 25 were considered. The three-dimensional flow velocities were invoked using SSIIM software. The distance between maximum scour hole depth and spur dike situation was fixed by changing the submergence ratio and equalled 0.6 times the flow height at the channel inlet. It was also observed that in the range of 30%–50% of the water level, in submergence percentage of 25, the changes in the separation zone length indicated a steep slope. The range of most changes in the separation zone for other submergence percentages was 5%–25% levels. The length of the separation zone changed between 0.48 and 2.88 and the length of the reattachment zone changed between 0.9 and 4.45 times the length of the spur dike wing. The value of the dimensionless distance of the centre of the upstream vortex at the half of the spur dike height level from the spur dike situation was fixed by changing the submergence percentage and was equal to 0.45 times the spur dike wing length.

합류부 하상고 불일치에 의한 두부침식 및 분리구역 특성분석

The pattern of headward erosion at tributary and the separation zone formation in a loosed bed at confluence according to the confluence angle, discharge ratio, and dredging depth ratio have been analyzed. The separation zone is defined the inside of zero velocity boundary at downstream of confluence. The limit of separation zone occurrence is presented with dredging depth ratio. The propagation length of knickpoint increases as the confluence angle, discharge ratio, and dredging depth ratio increase in general and its regression equation has been suggested. The length and width ratios of separation zone in a loosed bed increase as discharge ratio and confluence angle increase as well as in a fixed bed. The length ratio decreases and the width ratio increases as dredging depth ratio increases results in great increase of shape factor and backwater rise by the conveyance reduction at confluence. The regression equation of shape factor with confluence angle, discharge ratio, and dredging depth ratio has been suggested.

Investigation of cold-to-hot transfer and thermal separation zone through nano step geometries

Nanosteps form once nanochannels of various diameters connect to each other. The focus of this paper is to investigate the heat transfer and hydro/thermal field behavior in nanostep geometries using direct simulation Monte Carlo. The effects of the hydrodynamics separation on the pressure field and heat lines are reported, i.e., we show that the length of the hydrodynamics separation zone is different from the positive pressure gradient and thermal separation zones. Interestingly, cold to hot transfer is observed when the wall temperatures and inlet temperature are close to each other. We show that cold to hot heat transfer appears due to the interplay between the higher order term of the heat flux formula, which is a function of the second derivate of the velocity, with the Fourier term; the cold to hot transfer effect is omitted as the wall temperature or Knudsen number increases. In addition, the impact of different parameters, such as pressure ratio, Knudsen number, and wall temperature adjacent to the separation zone, are investigated. The dependence of the mass flow rate and the length of the separation zone on the wall temperature and the channel pressure ratio is considered. We show that Knudsen minimum is not observed in the step geometry for both isothermal and non-isothermal wall conditions.

The Effect of Support Structure on Flow Patterns Around T-Shape Spur Dike in 90° Bend Channel

Totally, spur dike is an economical way to preserve morphology of rivers. Numerical methods can be useful for evaluating hydraulic parameters for spur dike because of their reduced simulation time, while experimental ones take a long time and need lots of tools to simulate any models. In this paper, flow patterns around a T-shape spur dike and a support structure, which is located upstream of the T-shape spur dike, is analyzed in 90° bend channel by Flow-3D model. The numerical and laboratory data are compared in longitudinal section to verify numerical model. The results show very good correspondence between numerical and laboratory data. After verification numerical model, a support structure has been installed upstream of the T-shape spur dike with 3, 5, 7 and 9 times distance longer than the length of the T-shape spur dike. The support structure altered flow patterns and hydraulic parameters such as power of secondary flow and separation zone in all of sections. By increasing support structure distance from 3L up to 9L, the power of secondary flow around main spur dike decreases by 40–120% and the length of separation zone increases from 0.8 to 2.5 times bigger than the length of T-shape spur dike.

Experimental study of separation bubble control behind a backward-facing step using plasma actuators

In this paper, controlling flow around a backward-facing step by means of a dielectric barrier discharge plasma actuator is experimentally investigated. The pressure distribution on the step surface as well as the natural oscillations of the flow in the shear layer at three different Reynolds numbers from 18,000 to 54,000 (based on the step height) are measured in the wind tunnel. Then, by installing plasma actuators at different positions, the changes in pressure distribution and the length of the separation bubble are investigated. It was found that for controlling the separation bubble and expediting the flow recovery process the best position for installation of the plasma actuator is the upstream of the separation point i.e., step tip edge. Installing the actuator inside the separation bubble has a relatively low effect on the separation zone length. In addition, the most effective way for exciting the shear layer is utilizing the unsteady actuation with an exciting frequency equal to the natural frequency of the vortex shedding.